Chapter 13: Meiosis and Sexual Life (lammey)
13.2 Overview
Normal human somatic cells are diploid. They have 46 chromosome's made up of two sets of 23, one set from each parent. Human diploid cells have 22 homologous pairs of autosomes, and one pair of sex chromosomes; the latter determines whether the person is female (XX) or male (XY) In humans, ovaries and testes produce haploid genes by meiosis, each gamete containing a single set of 23 chromosomes (n=23). During fertilization, an egg and sperm unite, forming a diploid (2n =46) single-celled zygote, which develops into a multicellular organism by mitosis Sexual life cycles differ in the timing of meiosis relative to fertilization and in the points of the cycle at which a multicellular organism is produced by mitosis
Sex chromosomes
chromosome responsible for determining sex of individual (X and Y)
Genome
consists of the genes and DNA that make up the chromosomes inherited from our parents
Gametes
reproductive cells; they transmit genes from one generation to the next gametes from mom and dad unit passing genes from each parent to offspring
Sister Chromatids VS Two Chromosomes of a Homologous Pair
sister chromatids are two copies of one chromosome, associated together by cohesion --> together, they make one duplicated chromosome The two chromosomes of a homologous pair are individual chromosomes that were inherited from different parents --> homologs have different genes (alleles) at corresponding loci
Meiosis
A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell
Homologous chromosomes / homologs
A pair of chromosomes of the same length, centromere position, and staining patter that possess genes for the same characteristic at corresponding loci One homologous chromosome is inherited from the father, the other from the mother Chromosomes X and Y are exceptions to the general pattern of homologous chromosomes in human somatic cells --> only small parts of the X and Y are homologous
Where are genes located?
Each gene in an organism's DNA exists at a specific locus on a certain chromosome
How many chromosomes in a somatic cell?
Each somatic cell has 46 chromosomes --> two sets of 23 chromosomes (one from each parent)
Can haploid and diploid cells undergo both mitosis and meiosis?
Either haploid or diploid cells can divide by mitosis BUT only diploid cells can undergo meiosis because haploid cells only have a single set of chromosomes and cannot be further reduced
Behavior of Chromosomes Sets in the Human Life Cycle
Human life cycle begins when a haploid sperm fuses with a haploid egg *Fertilization* - union of haploid gametes to produce a diploid zygote; fusion of gamete's nuclei *Zygote* - fertilized egg (diploid) --> mitosis of the zygote generate the somatic cells of the body (both the chromosomes and genes from the zygote are passed to the somatic cells) Gamete formation involves meiosis --> this type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at fertilization
Independent assortment of chromosomes
Random orientation of pairs of homologous chromosomes at metaphase of meiosis I 50% change that a daughter cell of meiosis I will get the maternal chromosome or paternal chromosome of a homologous pair As consequence of independent assortment, each of us produces a collection of gametes differing greatly in their combinations of chromosomes inherited from parents In humans, 2^23 (8.4 million chromosome combos)
Three events unique to meiosis occur during meiosis 1
Synapsis and crossing over: during prophase 1; duplicated homologs pair up and cross over Homologous pairs at the metaphase plate: chromosomes are positioned at the metaphase plate as a pair of homologs in meiosis, rather than individual chromosome, as in mitosis Separation of homologs - at anaphase I of meiosis, the duplicated chromosomes of each homologous pair move towards opposite poles, but the sister chromatids of each duplicated chromosome remain attached (in anaphase of mitosis, sister chromatids separate)
Asexual VS Sexual Reproduction
*Asexual reproduction* - the generation of offspring from a single parent that occurs without the fusion of gamete (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases, the offspring are genetically identical to the parent; passing of all genes to offspring without fusion of gametes Only organisms that reproduces asexually have offspring that are exact genetic copies of themselves --> mitosis An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals or cells Genetic differences may occur in asexually reproducing organism as result of a mutations (horizontal gene transfer) *Sexual reproduction* - two parents give rise to offspring that have unique combinations of genes inherited from the two parents --> genetic variance Genetic variation is a result of chromosome behavior during the sexual life cycle
Inheritance of Genes
*Offspring acquire genes from parents by inheriting chromosomes* Inherited information is passed on in the form of each gene's specific sequence of DNA nucleotides Most genes program cells to synthesize specific enzymes and proteins which the produces inherited traits Replication of DNA allows copies to be passed from parent to offspring Most DNA is packaged in chromosomes within the nucleus (some is in the mitochondria and chloroplast - very small amount) One chromosome consists of a single long DNA molecule --> one chromosome has hundreds to thousands of genes, each of which is a specific sequence of nucleotides within the DNA molecule
Somatic Cells
Somatic cells - any cell in a multicellular organism except for a gamete or their precursors
Variation
differences between members of the same species
Genetics
the scientific study of heredity and heredity variation
Heredity
the transmission of traits from one generation to the next
Gene
unit of hereditary information consisting of a specific nucleotide sequence in DNA inherited from parents genes program specific traits
What does meiosis allow for?
Meiosis followed by fertilization ensures genetic diversity in sexually reproducing organisms and provides genetic variation that plays a role in natural selection.
What does meiosis do and what are the stages?
Meiosis reduces the number of chromosomes sets from diploid to haploid Meiosis is preceded by the duplication of chromosomes Duplication followed by meiosis I and meiosis II, resulting in 4 daughter cells, each with only half as many chromosomes as the parent cell *Meiosis I* - the first division of a two-staged process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell *Meiosis II* - The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosomes sets as the original cell
What causes genetic diversity
Mutations are the original source of genetic diversity --> changes in DNA create different versions of genes (alleles) --> shuffling of alleles during sexual reproduction produces variation
Meiosis Process 13.3 Overview
The two cell divisions of meiosis, meiosis I and meiosis II, produce 4 haploid daughter cells, The number of chromosome sets is reduced from two (diploid) to one (haploid) during meiosis I, the reductional division Meiosis is distinguished from mitosis by three events of meiosis I - Prophase I - Each homologous pair undergoes synapsis and crossing over between non-sister chromatids with the subsequent appearance of chiasmata - Metaphase I - chromosomes line up as homologous pairs on the metaphase plate - Anaphase I - Homologs separate from each other, sister chromatids remain joined at the centromere Meiosis II separates sister chromatids Sister chromatid cohesion and crossing over allow chiasmata to hold homologs together until anaphase I. Cohesins are cleaved along the arms at anaphase I, allowing homologs to separate, and at the centromeres in anaphase II, releasing sister chromatids
13.4 Overview
Three events in sexual reproduction contribute to genetic variation in a population: independent assortment of chromosomes during meiosis I, crossing over during meiosis I, and random fertilization of egg cells by sperm. During crossing over, DNA of non-sister chromatids in a homologous pair is broken and rejoined. Genetic variation is the raw material for evolution by natural selection. Mutations are the original source of this variation; recombination of variant genes generates additional genetic diversity.
Meiosis VS Mitosis
*DNA Replication* Mitosis - interphase Meiosis - interphase before meiosis I *Number of Division* Mitosis - One Meiosis - Two *Synapsis of homologous chromosomes* Mitosis - does not occur Meiosis - Occurs during prophase I along with crossing over between non-sister chromatids; results in chiasmata that hold airs together due to sister chromatid cohesion *Number of daughter cells and genetic composition* Mitosis- 2, each genetically identical with the same # of chromosomes Meiosis - 4, each haploid and genetically different *Role in the animal or plant body* Mitosis - allows multicellular organisms to arise from a single cell; produces cells for growth, repair, and asexual reproduction; produced gametes in the gametophyte plant Meiosis - produces gametes (in animals) or spores (in sporophyte plants); reduced number of chromomes sets by half and introduces genetic variability Meiosis reduced the number of chromosome sets from diploid to haploid while mitosis conserves the number of chromosome sets
Variety of Sexual Life Cycle
Although the alternation of meiosis and fertilization is common to all organisms that produce sexually, the timing of these two events in the life cycle varies depending on the species (three main life cycles) 1)Gametes are the haploid cells --> fertilization --> diploid zygote that divides by mitosis; humans and animals 2) Alternation of generations - a life cycle in which there is both a multicellular diploid form (the sporophyte) and a multicellular haploid form ( the gametophyte) --> characteristic of plants and some algae Meiosis in the sporophyte produces haploid cells called spores -->spore divides mitotically, generating a multicellular haploid gametophyte -->cells of the gametophyte give rise to gametes by mitosis --> fusion of two haploid gametes at fertilization results in a diploid zygote, which develops into the next sporophyte generation 3) Haplontic -After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing --> meiosis produces haploid cells that then divide by mitosis and give rise to either unicellular organisms or a haploid multicellular adult organism --> the haploid organisms carry out mitosis, producing cells that develop into gamete -->fungi, some protists, some algae
Origins of Genetic Variation Among Offspring
Behavior of chromosomes during meiosis and fertilization is responsible for most variation 3 mechanisms contribute to genetic variation: independent assortment of chromosomes, crossing over, and random fertilization
Crossing over
Crossing over creates recombinant chromosome (a chromosome created when crossing over combines DNA from two parents into a single chromosome) Crossing over produces chromosomes with new combos of maternal and paternal alleles Different arrangements of non-identical sister chromatids during meiosis II further increases the number of genetic variations
Crossing Over and Synapsis
During Prophase I of meiosis After interphase, chromosomes have been duplicated --> sister chromatids held together by cohesins Early in prophase one, the two members of a homologous pair associate loosely along their length --> each gene on one homolog is aligned with the corresponding gene on the other The DNA of the two non-sister chromatids (mom and dad) is broken Then, zipper-like structure called the *synaptonemal complex* is formed --> it attaches one homolog to the other. When the synaptonemal complex is fully formed, the two homologs are said to be in *synapsis* During synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the non-sister chromatid, producing *crossovers* (paternal chromatid is joined to a piece of maternal chromatid) Points of crossing over become visible as *chiasmata* after the synaptonemal complex disassembles and the homologs move apart but remain attached (sister chromatids still attached even though some DNA is not with original chromosome) Chiasmata hold homologs together as the spindle forms for the first meiotic division
Evolutionary Significance of Genetic Variation within populations
Individuals best suited to environment have the most offspring, transmitting their genes more Natural selection results in the accumulation of genetic variations favored by the environment New combos of alleles may be better suited for environment
Meiosis I VS Meiosis II
Meiosis I : reductionional division (reduces chromosomes from diploid to haploid Meiosis II : equational division (sister chromatids separate, producing haploid daughter cells)
Random fertilization
The fusion of a male gamete with a female gamete during fertilization will produce a zygote with 70 trillion diploid combos (2^23 x 2^23)
Diploid VS Haploid
The number of chromosomes in a single set is represented by n *Diploid cell* - cell with two chromosomes (2n), one set inherited from each parent (somatic cells are diploid) In a cell where DNA synthesis occurred and chromosomes are duplicated with 2 sister chromatids, still diploid with duplicated chromosome's because still only has two sets of information *Haploid cells*- a cell containing only one set of chromosomes (n=23); gametes
Autosome
a chromosome that is not directly involved in determining sex
Karyotype
a display of the chromosome pairs of a cell arranged by size and shape
Locus
a genes specific location along the length of the chromosome
Life Cycle
the generation to generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring
